It is proposed to investigate the biosynthesis of three naturally occurring cyclopentanoid compounds. These are the alkaloids cephalotaxine and sesbanine and the antibiotic dehydropentenomycin. It is hypothesized that the cyclopentane ring in each of these compounds arises by contraction of a six-membered ring. Evidence in favor of this hypothesis has already been deduced for cephalotaxine, but the nature of the intermediates involved in the ring contraction step has yet to be elucidated. The biosynthesis of sesbanine and dehydropentenomycin is at present unexplored and the hypothetical ring contractions require verification. The major objectives of the work are: a) to synthesize and evaluate the potential intermediates in the ring contraction leading to cephalotaxine, and b) to establish the biogenetic origin of the carbon atoms in the skeletons of sesbanine and dehydropentenomycin. The method to be utilized is the administration of specifically labeled precursors to the appropriate organisms followed by location of the labels in the natural products via degradation or c.m.r. spectrometry.